In the field of NMR spectroscopy, a sample is surrounded by an NMR probe that consists of a radio frequency (RF) coil tuned to generate a field at a desired excitation frequency and receive a return NMR signal. More complex probes will generate multiple frequencies so as to excite the nuclei of more than one different element in the sample (e.g., hydrogen nuclei 1H (proton) and fluorine nuclei 19F). These “double resonance” probes (in the case of a probe generating two separate frequencies) and “triple resonance” probes (in the case of a probe generating three separate frequencies) have been used for many years, with varying degrees of success. One of the problems faced by multiple resonance probes arises when two of the “resonances” are closely spaced in frequency. For two resonances that have a relatively wide frequency separation (such as 1H and 13C), it is fairly easy to isolate the two frequencies in the generation circuit. However, for elements having resonant frequencies closer together (such as 1H and 19F), it becomes more difficult to get good frequency discrimination between them.
In systems having a single sample coil, it is necessary to generate each desired resonant frequencies and apply them to the coil, so the frequency isolation must come from the circuits themselves. Different approaches have been used to try to better isolate resonances that are close in frequency. In U.S. Pat. No. 6,307,371, a design is used for close, high frequency resonances that has come to be known as “overcoupling.” In this design, a sample coil is provided that has electrically coupled to one side circuitry associated with lower frequency channels. The circuit for two high frequency channels is connected to the other side of the sample coil, and consists of a single section of transmission line with an inductor at a position along its length that results in the formation of two inductively coupled quarter-wavelength (λ/4) resonators, each of which is tuned to a different one of the high frequencies. The inductor is typically made adjustable, along with a trimmer at one end of the transmission line, so that the two resonant halves may be precisely tuned. Although this design successfully applies both high resonant frequencies to the sample coil, it is inherently unbalanced, in that the peak of the magnetic field distribution is not in the center of the sample coil for both the high frequency and the low frequency resonances. In addition, the circuit necessary to support this design, if made small enough to fit within the standard-sized bore of a NMR magnet, would be relatively inefficient.
Another “overcoupled” circuit is shown in U.S. Pat. No. 4,742,304. In this arrangement, the resonance of a sample coil tank circuit (consisting of the sample coil and a first capacitance) is split into two closely-spaced frequencies by coupling it to a second, “dummy” tank circuit. While this circuit can be effective for lower frequencies, it does not achieve the necessary separation between resonances at higher frequencies (i.e., above 400 Mhz).
In U.S. Pat. No. 5,861,748, a double resonance circuit uses a highly branched assembly of coaxial transmission lines of different lengths and different branch points having each distinct matching elements for the various measuring frequencies. However, in this design, all of the channels are on the same side of the sample coil, making the circuit inherently unbalanced. In addition, the circuit is physically very large, making it difficult to fit in a standard bore.
In Methods for the Analysis and Design of a Solid State Magnetic Resonance Probe, Review of Scientific Instruments 69(9) 1998, the authors describe a probe for use with multiple resonances, including the closely spaced resonant frequencies for 1H and 19F. The probe circuit makes use of transmission lines and capacitors to form the desired nodes, and to provide a probe circuit with higher efficiencies than previously achieved. However, the probe requires a high number of transmission lines very strategically placed around the sample coil.